Many RF communications systems in use today require wide spectra. It is not uncommon for some systems to have RF signals that have a bandwidth ratio of more than 2:1. If the RF signals in such a system need to be up-shifted or up-converted in frequency, the RF signals are usually up-converted by heterodyning them (multiplying) by another signal at a higher or lower frequency.
One problem associated with wideband signals (e.g., signals with a bandwidth of more than one octave) for RF communication systems is the generation of second order distortion (e.g., second order spurs). For a first signal of fundamental frequency f1, second order spurs of intermodulation distortion (IM2) may be created by 1) 2nd order harmonics of the first signal at twice the frequency (i.e., 2f1) and/or 2) mixing the first signal with a second signal of fundamental frequency f2, where the spurs occur at frequencies of f1-f2 or f1+f2.
One way to improve second order distortion of an amplifier is to increase its DC power. However, increasing the DC power will increase thermal energy dissipation, which may cause problems in the system such as overheating of electrical components. Thermal energy dissipation may be an important system design consideration for phased array systems where many active electronics are used.
As used herein, a “differential signal” is a signal that propagates through pairs of conductors. At any given instant, the voltages on a pair of conductors carrying a differential signal are equal in amplitude but opposite in polarity. For example, the signals on conductors carrying a differential signal are equal and opposite to each other such that the algebraic sum of the signals on the two conductors is substantially equal to zero. A “differential signal” is also known in the art as a “common mode” or a “double-ended signal” signal. A “single-ended” signal on the other hand is one that travels along a conductor where the voltage on the conductor swings above and below zero volts. Single-ended signals are therefore normally measured with respect to a reference or ground potential whereas a differential signal is measured or referenced with respect to its paired signal conductor.
Differential electronics may provide cancellation (20 to 30 dB is typical) of second order distortion. Examples of differential electronics comprise a push-pull or differential amplifier. Baluns are widely used in electrical and electronic engineering for the purpose of converting a balanced input to an unbalanced output or vice versa. In radio frequency (RF) and microwave monolithic integrated circuits (MMICs), baluns may be used for designing, for example, push-pull low-noise amplifiers (LNA) or double balanced mixers. The magnitude of the second order spur cancellation is dependent upon amplitude and phase errors due to baluns used in the differential electronics. As is well-known, it is very difficult to design a wideband MMIC balun with a bandwidth ratio higher than (3:1) with good phase and amplitude balance. As is well-known, a wideband balun has a higher insertion loss, which will degrade a Noise Figure (NF) of the system if it is used at the input. In addition, the size of the multi-octave balun is usually large.
As is known in the art, multiplication of a first input sine (or cosine) wave with frequency f1 with a second input sine (or cosine) wave with frequency f2 yields two output sine wave signals, for example, first and second output sine waves (or cosine). A frequency of the first output sine wave is equal to a sum of the first and second input sine waves. A frequency of the second output sine wave is equal to a difference between the first and second input sine waves. For example, where the first input comprises an RF signal at 100 MHz and the second input comprises an RF signal at 2.0 GHz, the first output signal comprises a frequency of 2.1 GHz and the second output signal comprises a frequency of 1.9 GHz.
In a wideband communication system, such as one that uses signals between 100 MHz. and 1.0 GHz., a prior art method for suppressing harmonics in up-converted signals is to split a wide baseband spectrum into several different slices or ranges. For example, 100 MHz. to 200 MHz., 200 MHz to 300 MHz., 300 MHz.-400 MHz., etc. Since each band pass filter will allow only a portion of the RF to pass, each filter will suppress 2nd, 3rd etc. harmonic signals that might be present in the RF signal.
The output of each band-pass filter is summed together to produce a reasonably close facsimile of the RF signal. Note that the above filtering of the spectrum into multiple slices may or may not be followed by frequency translation.
One unavoidable problem with using band-pass filters is that each filter will have at least some “roll-off” at each of end of its nominal pass band. RF signals at or near the cut-off frequencies of a filter will be somewhat attenuated. If a RF signal at or near a filter's cut-off frequency is weak, the attenuation caused by the filter's roll-off might cause the signal to be lost. Put another way, when multiple filters are used to split up a wide spectrum into discrete slices, the filter roll-off of each filter will cause the resultant signal (which may or may not be frequency translated) to have “blind spots” that are centered at each base-band filter's nominal cut-off frequencies. Because the “blind spots” caused by separate band-pass filters are unusable, their bandwidth is wasted. As the demand for RF communications grows, RF spectrum becomes more precious.
Even if blind spots were not a problem, using such a method requires additional costs associated with having to use multiple filters, multiple amplifiers, and multiple mixers. As is well-known, as parts count increases, size, weight, and cost also increase.